Links - Geoscience Society of New Zealand
Links - Geoscience Society of New Zealand
Links - Geoscience Society of New Zealand
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N<br />
0 1 2 km<br />
Fig. 1. Bathymetry <strong>of</strong> Lake Rotorua, including elevation <strong>of</strong> the surrounding landscape.<br />
Note the historical lake bed (shaded mauve), the current lake extent (delineated by the<br />
brown-shaded region <strong>of</strong> coarse sediments within the historical lake bed) and the transition<br />
to fine sediments <strong>of</strong> the inner lake basin (blue regions in the interior). Mokoia Island is<br />
readily distinguished near the centre <strong>of</strong> the present lake basin.<br />
Pock marks and methane eruptions<br />
Early attempts to use seismic techniques to survey the<br />
stratigraphy <strong>of</strong> the lake sediments (Davey, 1992) were<br />
unsuccessful due to the absorption <strong>of</strong> acoustic signals by<br />
gas within the sediments. This gas was interpreted to be<br />
<strong>of</strong> geothermal origin. A recent seismic survey (Pearson,<br />
2007) also showed no structural features <strong>of</strong> the lake<br />
sediments except where there are small, circular, flatbottomed<br />
depressions, detectable in Fig. 1 as minute<br />
depressions in the diatomaceous ooze <strong>of</strong> the deeper bed<br />
<strong>of</strong> the lake. The depressions are typically steep-sided and<br />
up to 5 m deeper than the surrounding lake bed, and<br />
Fig2. Gas erupting on the<br />
around 50 m in diameter. Similar circular depressions surface following sediment<br />
are common in shallow marine and lacustrine sediments<br />
and have been termed pockmarks, with their formation<br />
disturbance by gravity corer.<br />
interpreted as a result <strong>of</strong> gas discharge (Rodgers et al.,<br />
2006). Evidence <strong>of</strong> ebullition <strong>of</strong> gas can commonly be observed at the surface <strong>of</strong> Lake Rotorua<br />
on calm days (Fig. 2), particularly over pockmarks, and also in sediment cores raised to the<br />
lake surface (Pearson, 2007). Gas captured from beneath the water surface was a mixture <strong>of</strong><br />
GSNZ <strong>New</strong>sletter 143 (2007) Page 8